Disc drive memory systems are being utilized in progressively more environments besides traditional stationary computing environments. Recently, these memory systems are incorporated into devices that are operated in mobile environments including digital cameras, digital video cameras, video game consoles and personal music players, in addition to portable computers. These mobile devices are frequently subjected to large magnitudes of mechanical shock as a result of handling. As such, performance and design needs have intensified including improved resistance to a shock event, improved robustness and reduced power consumption.
Disc drive memory systems store digital information that is recorded on concentric tracks of a magnetic disc medium. At least one disc is rotatably mounted on a spindle, and the information, which can be stored in the form of magnetic transitions within the discs, is accessed using read/write heads or transducers. A drive controller is typically used for controlling the disc drive system based on commands received from a host system. The drive controller controls the disc drive to store and retrieve information from the magnetic discs. The read/write heads are located on a pivoting arm that moves radially over the surface of the disc. The discs are rotated at high speeds during operation using an electric motor located inside a hub or below the discs. Magnets on the hub interact with a stator to cause rotation of the hub relative to the stator. One type of motor has a spindle mounted by means of a bearing system to a motor shaft disposed in the center of the hub. The bearings permit rotational movement between the shaft and the sleeve, while maintaining alignment of the spindle to the shaft. The read/write heads must be accurately aligned with the storage tracks on the disc to ensure the proper reading and writing of information.
A demand exists for increased storage capacity and smaller disc drives, which has led to the design of higher recording areal density such that the read/write heads are placed increasingly closer to the disc surface. Because rotational accuracy is critical, disc drives currently utilize a spindle motor having fluid dynamic bearings (FDB) between a shaft and sleeve to support a hub and the disc for rotation. In a hydrodynamic bearing, a lubricating fluid provides a bearing surface between a fixed member and a rotating member of the disc drive. Hydrodynamic bearings, however, suffer from sensitivity to external loads or mechanical shock. Fluid can in some cases be jarred out of the bearing by shock events. An embodiment of a FDB motor includes a magnetically biased motor wherein the bearing design cooperates with a magnetically biased circuit or element to establish and maintain fluid pressure in the bearing areas by providing an axial magnetic force, especially in designs where the thrust bearing is defined in the gap at the end of the shaft. Typically in such systems, however, the only force or structure holding the rotating portion of the motor in place is the axial magnetic force; therefore, if shock axial forces exceed magnetic forces in the motor, the rotor can shift and the disk drive can become damaged or fail. Accordingly, FDB spindle motors, and particular, those having electromagnetic bias and a single thrust bearing, generally include features to limit the axial displacement of the rotating portions relative to the stationary portions during a shock event. Often such features are referred to as a “shock limiter.” A limiter generally limits or reduces the potential for axial displacements of the rotating portions of the motor relative to stationary portions beyond a desired or acceptable range of axial motion.
The hydrodynamic bearing life of motors used in disc drives is limited by lubricant evaporation. A sufficient amount of lubricant such as oil must be maintained in a capillary seal reservoir to offset evaporation losses. The evaporation rate is further accelerated when special low viscosity oils are used to reduce power. The lower viscosity oils generally have a higher rate of evaporation. If a shock event occurs with a motor having an insufficient volume of lubricant, rotating surfaces may come in direct contact with stationary portions. The dry surface-to-surface contact may lead to particle generation or gall and lock-up of the motor during contact. Particle generation and contamination of the bearing fluid may also result in reduced performance or failure of the spindle motor or disc drive components.
Additionally, the maximum amount of oil that can be filled in the capillary seal is limited by shock requirements, since oil tends to shift and leak out of the seal when shocked. In addition to maintaining a sufficient amount of oil in the seal reservoir to account for evaporation losses, the minimum amount of oil that can be filled in the capillary seal must generally also account for cold temperature contraction of the oil, fill process tolerances, and the volume of oil that recedes into the motor bearing cavities when the axial play gap opens. The requirement of accounting for the axial play volume is intended to avoid allowing the seal meniscus from receding into the motor and trapping air inside the bearing where it poses a reliability risk. Also, as axial height of spindle motors is reduced, the spacing between bearing components decreases, thereby minimizing angular or rocking stiffness of the bearings. As hydrodynamic bearing motor requirements call for lower power, higher stiffness and longer life, there is a need for a capillary seal and an axial limiter design that purges air and reduces power while enabling higher stiffness and longer life.